Why Britain requisitioned foreign warships under construction

Did Britain normally requisition foreign ships being built in its yards when war broke out?

Yes in major wars when suitable hulls were present; Britain used them to expand the Royal Navy quickly and deny them to a possible enemy.

The Turkish submarines discussed just before this exchange had initially been requisitioned by Britain and were delivered later when supporting Turkey became strategically useful. Taking foreign orders in British yards was standard major-war practice despite peacetime assurances. The policy rapidly enlarged the Royal Navy and prevented a completed ship from becoming available to a possible enemy. During the Falklands War, no comparable foreign warships were under construction in British yards to take over. The policy illustrates the sovereignty risk a smaller navy accepts when it depends on a foreign belligerent's shipyards.